1. Field of the Invention
The present invention relates to an error detection apparatus for use with electrophotographic image forming apparatuses such as copiers, facsimile machines and printers, the error detection apparatus preventing image defects and damage to component parts attributable to a failure of the toner sensor for detecting the presence or absence of toner in the developing unit of the apparatus.
2. Description of the Related Art
One typical image forming apparatus is the remote printer that continuously records image data 24 hours a day, the data being received from a host computer or the like. This kind of printer has its toner housing filled with as much toner as can be accommodated so as to make a large number of copies or prints on a continuous basis. In such a setup, the developing unit of the printer incorporates a toner sensor that detects the presence or absence of toner. When the toner sensor detects the absence of toner, a display unit attached to the printer is activated to display a prompt message for toner replenishment. At the same time, the remaining toner in the developing unit is utilized to continue the image recording operation. When a predetermined number of prints has been completed, the printer is brought to a stop.
As shown in FIG. 2, the detection signal from the toner sensor is typically made of a high level signal 100 and a low level signal 101. The high level signal 100 is output when toner is detected on the surface of the toner sensor; the low level signal 101 is output when toner is found absent. The high or low level signal 100 or 101 is not merely output when toner is found present or absent in the developing unit; such signals may also be triggered erroneously. A sensor cleaning member is provided which operates in synchronism with an agitator to keep the sensor surface free of toner. In this arrangement, a rotating agitator operates the sensor cleaning member concurrently, often causing the toner sensor to detect the presence or absence of toner erroneously. Furthermore, since toner is consumed every time a print or a copy is made, a subsequent toner-replenishing action can cause the toner sensor to detect the presence or absence of toner incorrectly.
One solution to the above problem is the system proposed in Japanese Patents Laid-open Nos. Hei 1-288876 and Hei 3-267966. The proposed system involves checking the signal from a toner sensor arrangement at intervals of 100 ms. This system defines as a toner-absent state the state in which the low level signal 101 indicating the absence of toner is output continuously for 1000 ms or longer. As depicted in FIG. 14, the proposed system comprises three different means: counting means for counting the number of prints made while the toner-absent state is being detected by the toner sensor arrangement (steps 1.fwdarw.2.fwdarw.3.fwdarw.4.fwdarw.5.fwdarw.6.fwdarw.7.fwdarw.8.fwdarw.10 .fwdarw.6): subtracting means for subtracting from the above count value the number of prints made after the toner sensor arrangement has stopped detecting the absence of toner (steps 6.fwdarw.7.fwdarw.8.fwdarw.9.fwdarw.2); and stopping means (step 16) for stopping the machine when the number of prints made has reached a predetermined count value (step 6).
The developing unit of the conventional system above has its toner sensor arrangement composed of a first and a second toner sensor. The first toner sensor detects the absence of toner in a developer housing, and the second toner sensor detects the absence of toner in a toner box. When the first toner sensor detects a toner-absent state, a toner dispensing motor is driven until the developer housing is replenished sufficiently with toner. A toner-present state being detected means that the developer housing contains a sufficient amount of toner. In this state, the toner dispensing motor is held inactive.
Everything is fine as long as the toner sensors work properly for toner detection. Problems arise if a toner sensor itself fails; if a sensor cleaning member peels, breaks or bends, causing the toner sensor to act erroneously; or if any harness is severed or any connector is unplugged. Illustratively, the first toner sensor itself can fail either electrically due to static electricity (charges accumulated in toner triggering a leak), or mechanically (sensor surface damaged by a deformed cleaning member). A toner cleaning member being peeled, broken or bent can leave toner deposited on the sensor surface. This may cause the first toner sensor incorrectly to detect the presence of toner even though the developer housing does not have enough toner inside. A severed harness or an unplugged connector may be brought about inadvertently by the user or service engineer servicing the developing unit or the like.
If any of the troubles above occurs, the first toner sensor incorrectly detects either the presence or the absence of toner. Erroneous detection of two cases is possible by use of appropriate software. One of the two erroneously detectable cases is one in which the presence of toner is continuously detected even though the developer housing actually does not have a sufficient amount of toner inside. In this case, the toner dispensing motor remains inactive and no toner is supplied. As a result, the residual toner is consumed by the ongoing copy or print operation. Soon the copies or prints coming out of the machine have blank portions; eventually nothing is copied or printed on the paper.
The other erroneously detectable case is one in which the absence of toner is continuously detected even though the developer housing contains toner inside. In this case, the toner dispensing motor remains active so that the housing is replenished with excess toner. Soon the developer housing is filled with toner; eventually excess loads either damage the toner dispensing motor or destroy the developer housing. In either case, the quality of copies or prints drops drastically, or mechanical failure such as a damaged motor or a destroyed developer housing results.
The same applies to the second toner sensor on the toner box side. If the second toner sensor fails, the presence of toner is erroneously detected in the toner box. Thus the prompt message for toner replenishment does not appear even though the toner cartridge has no toner inside. The toner sensor then detects a toner-absent state and issues orders to drive the toner dispensing motor. But toner is not supplied, with the result that subsequent copies or prints include growing blank portions. Eventually nothing is copied or printed on the paper. The same eventuality can also happen if the toner dispensing motor fails. If the toner dispensing motor stops running because of a severed harness or an unplugged connector, no toner is supplied to the developer housing. Soon the copies or prints coming out of the machine have blank portions; eventually nothing is copied or printed on the paper.